Embodiments of the present invention relate to a calibration device, a signal processing device for an HF signal and to a user interface for the signal processing device as well as a method for calibrating and operating the same.
Signal processing devices are, for example, channel or payload simulators where an HF signal (high frequency signal) is fed in and output again in modified form. The modifications applied to the HF signal correspond approximately to the signal changes that would result when transmitting the HF signal under real conditions via a channel (such as a telecommunication channel or a satellite connection) due to spurious influences. In that way, such signal processing devices can simulate how an HF signal would reach the receivers after transmission via a channel (signal path). In the following, such a signal processing device or such a channel simulator according to conventional technology will be discussed based on FIG. 3.
FIG. 3 shows a channel simulator 10 having an input stage 12, a signal processing stage 14 and an output stage 16. The input stage 12 can, for example, comprise an analog-to-digital converter, a signal mixer, amplification elements and attenuation elements (not shown). Analogously, the output stage 16 comprises a digital-to-analog converter and further amplification or attenuation elements (not shown). Since signal processing is performed in a digital manner, i.e., after converting a real power (in dBm) into a digitally represented number (dB), the signal processing device 14 is implemented in the form of a digital addition mixer.
For simulating a transmission path with the signal simulator 10, the input stage 12 is provided with an ideal HF signal 18 (payload signal), which is digitized with the analog-to-digital converter 12a. In the signal processing stage 14, the digitized payload signal is modified according to a modification signal 20, or subject to different spurious influences, such as noise, nonlinear distortions, linear distortions (filter), or amended according to a channel module for multi-path propagation. For this, for example, the payload signal is superimposed by the modification signal 20 and subsequently converted again into an analog HF signal in the output stage 16 and output as simulated signal 22 (including the modification portions 20). Since the signal processing device 10 only has a limited control range, typically, the level of the payload signal 18 is adapted in the input stage 12 or in the output stage 16 by means of amplification or attenuation elements. Thus, the payload signal 18 can be transmitted from the input to the output with an adjustable defined amplification (after calibration). Amplification adaptation of the payload signal 18 is performed, for example, such that during analog-to-digital conversion in the analog-to-digital converter of the input stage 12, and during digital-to-analog conversion of the combined payload signal in the output stage 16, a good signal/noise ratio is obtained. Here, it has to be noted in particular that the power of the sum signal during signal processing (cf. signal processing stage 14) of the payload signal 18 can and should normally be amended.
Here, the approximate amplification of the input stage 12 and the output stage 16 is adjusted by the user. An exact control of the signal level is only possible with the help of external measurement equipment. Thus, the user can only indirectly determine whether the selected amplification adjustment is appropriate. A further conventional solution is the automatic gain control (AGC) illustrated in FIG. 3. The same is arranged between input stage 12 and signal processing stage 14 and thus regulates the amplification of the payload signal 18 in the digital domain. For this, the automatic gain control 13 comprises a multiplication mixer 13a and an amplification element 13b which is implemented to measure the digitized payload signal 18 (power detection) and to compare the same to internally generated signals. In the next step, the amplification element 13b provides the mixer 13a with an amplification signal, such that the digitized payload signal can be adapted. For controlling the adaptation, the signal for power detection is typically branched off after the mixer 13a in the amplification element 13b such that a feedback loop is formed in the digital domain. In other words, the mixer 13a of the automatic gain control 13 changes the digital values representing a real power (e.g., in dBm) such that no amplification in the classical sense takes place, but only conversion of the digital signal values for the HF signal 18. Switching the amplification or provision with an amplification signal mostly results in an undesirable interference of the payload signal 18. Further, undesirable cross sensitivities result in the sense that, for example, adding a modification signal 20 simultaneously reduces the power of the payload signal at the output. Further, the control of the input hardware is not at an optimum, since the correct control is only adapted subsequently in a digital manner and spurious signals as well as noise levels contribute to the amplification. All internally generated signals 20 which were added to the payload signal 18, corrupt the output power (which has been determined, for example, prior to determining a test signal). Thus, the user has to subsequently calculate level changes by the modification signal 20, since the signals 20 additionally added by the user are not considered when setting the amplifications. Thus, there is a need for an improved approach.